[linux-lvm] re --corelog: does synchronization re-copy all data?

[linux-lvm] re --corelog: does synchronization re-copy all data?

[Daniel B.]

In the LVM Administrator's Guide, at least the one at
http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/LV_create.html ,
it says:"

   LVM maintains a small log which it uses to keep track of which regions
   are in sync with the mirror or mirrors. By default, this log is kept
   on disk, which keeps it persistent across reboots. You can specify
   instead that this log be kept in memory with the --corelog argument;
   this eliminates the need for an extra log device, but it requires that
   the entire mirror be resynchronized at every reboot.

Does that last sentence mean that _all_ the data is recopied, or just
that all _regions_ are checked and not all the data necessarily has
to be copied (perhaps only change regions have to be copied, but
unchanged regions don't)?



More generally, can LVM perform RAID1-style mirroring with only
two disks (without having to re-copy everything each boot)
(and, of course, being recoverable in case either disk fails)?



Thanks,
Daniel
-- 
Daniel B.
dsb@smart.net

Re: [linux-lvm] re --corelog: does synchronization re-copy all data?

[malahal]

Daniel B. [dsb@smart.net] wrote:
> In the LVM Administrator's Guide, at least the one at
> http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/LV_create.html ,
> it says:"
>
>   LVM maintains a small log which it uses to keep track of which regions
>   are in sync with the mirror or mirrors. By default, this log is kept
>   on disk, which keeps it persistent across reboots. You can specify
>   instead that this log be kept in memory with the --corelog argument;
>   this eliminates the need for an extra log device, but it requires that
>   the entire mirror be resynchronized at every reboot.
>
> Does that last sentence mean that _all_ the data is recopied, or just
> that all _regions_ are checked and not all the data necessarily has
> to be copied (perhaps only change regions have to be copied, but
> unchanged regions don't)?

Everything has to be re-copied because it doesn't know what is changed
as the "log" is kept in memory which is lost after a reboot.

> More generally, can LVM perform RAID1-style mirroring with only
> two disks (without having to re-copy everything each boot)
> (and, of course, being recoverable in case either disk fails)?

You should be able to force LVM to create a log device on one of your
two devices (--alloc anywhere)

Thanks, Malahal.

Re: [linux-lvm] re --corelog: does synchronization re-copy all data?

[Daniel B.]

malahal@us.ibm.com wrote:
> Daniel B. [dsb@smart.net] wrote:

>> More generally, can LVM perform RAID1-style mirroring with only
>> two disks (without having to re-copy everything each boot)
>> (and, of course, being recoverable in case either disk fails)?
> 
> You should be able to force LVM to create a log device on one of your
> two devices (--alloc anywhere)

So what happens if the disk containing the log dies?  Does LVM
keep running (presenting the logical volume implemented on the
physical volumes on the still-working disk)?

(And what happens at reboot time (with the failed disk still in
place)?)

Thanks,
Daniel
-- 
Daniel B.
dsb@smart.net